In most fluid systems, there is a need to guard against damage associated with pressure surges. Typically, a pressure surge is generated when there is a change in the rate of flow of fluid in a closed conduit. The surge pressure can be dangerously high if the change in the rate of fluid flow in the conduit is too great. In many applications, such as pipelines and storage or loading and unloading terminals, there is a need to protect equipment and personnel from the potential damages that such pressure surges create.
Pressure surges are sometimes called “water hammer.” The surge of pressure can be generated by any pipeline component that causes the fluid velocity in the conduit to change. For example, surge pressures or water hammer can be created by closing an automatic emergency shut down (ESD) device, the closure or opening of a manual or power operated valve, slamming shut of a non-return valve, or starting or stopping a pump. To protect larger fluid systems from piping component failure, the pressure surge associated with the water hammer must be relieved. In piping systems, it is especially important that a surge relief system be adaptable for a quick response time, and adaptable with respect to high flow capacity.
Surge pressures may vary in magnitude from virtually undetectable to such severity as to cause significant problems. Several examples of problems caused by insufficient surge protection in fluid systems include separation of flanges, pipe fatigue, weld failure or circumferential or longitudinal over stressing of the pipe, pumps knocked out of alignment, severe damage to piping and piping supports as well as damage to specialized components such as loading arms, hoses, filters and the like due to the hydraulic shock propagated through the fluid. It is important that during interruption of steady-state operation a potentially damaging transient, i.e., a water hammer, is detected, and automatically expunged by relieving a sufficient volume of fluid from the system, thereby attenuating the transient to within acceptable limits.
Typically, protection is provided by a fixed-set point surge relief device. A fixed-set-point surge relief system provides that when the increase in pressure reaches a specific set pressure level, a valve or valves open to relieve the excess pressure and attenuate the transient.
Alternatively, a floating-set-point surge relief system provides that when the time rate of change of pressure exceeds a pre-determined value, a valve or valves open to relive the excess pressure and control the pressure transient. An important feature of the floating-set-point system is that it provides protection from pressure surges even through the steady-state fluid pressure level in the pipeline may change due to varying sets of operating conditions. In such situations, a surge relief system must respond rapidly yet operate very smoothly Such a system should respond to the increasing pressure rise, (i.e., the transient pressure rise), and timely open the pressure relief mechanism. Thereafter, the system should control the rate of pressure rise, (i.e. the transient) to maintain the pressure within acceptable limits. The relieved flow can be dissipated in a large storage vessel and later returned to the product line.
The above-described surge relief systems have drawbacks however. While these systems prevent excess pressure within the pipeline, they do not address the unbalanced pipeline thrust forces or transients that result from the initial pressure surge. And while others address both the excess pressure within a pipeline along with the transients, they unnecessarily discharge fluid from the pipeline in response to transients of brief duration or pressure variations within normal range of pipeline operation, which can affect efficiency and/or become a nuisance.
Accordingly, it is desirable to provide a surge relief method and apparatus that prevents the likelihood of unnecessary discharge of fluid from a pipeline. Moreover, it is desirable to provide a surge relief method and apparatus that prevents likelihood of the discharge of fluid when the pressure variations within the pipeline have a magnitude less than a prescribed value and that ignores any pressure transient unless the positive rate of rise is in excess of a specific value.